Water-in-salt electrolytes extend the voltage stability range beyond that of dilute systems, enabling the use of high-voltage materials in aqueous energy storage. This stability is often attributed to the formation of a solid electrolyte interphase (SEI) or reduced water activity. However, by studying the hydrogen evolution reaction (HER) on platinum in NaClO4 electrolytes (1–17 molal) using electrochemical measurements, MD simulations, and DFT calculations, we show that alternative mechanisms strongly influence the stability window. Specifically, we disentangle the effects from water activity, the local pH increase, and kinetic and transport limitations arising from disrupted hydrogen bonding and sluggish water transport. We observe a nearly linear relationship between the decrease in the surface water coverage and HER exchange current density. As a result, the HER kinetics is slower: with a 7 times decrease in the exchange current density and a 1.5 times increase in the Tafel slope in 17 m solution compared to 1 m. Our MD simulations further reveal that sluggish water transport within the double layer significantly limits the HER, extending the experimental stability window. Ultimately, in this non-SEI-forming electrolyte, reduced bulk water activity plays only a relatively minor role in enhancing the stability of water-in-salt systems.
Vazquez et al. (2025) studied this question.